BayCEER Workshop 2023

Fe(II)-catalyzed ferrihydrite transformation in the presence of phosphate during redox oscillations: mutual effects on iron mineralogy and phosphate sorption

Phosphate serves as a vital nutrient for plant growth but is also a contributor to eutrophication. Its environmental behavior is often connected to its adsorption onto iron (Fe) oxides, a process significantly impacted by the transformation reactions of Fe oxides. In environments characterized by suboxic and anoxic conditions, there is a facilitation of the transformation of ferrihydrite into more crystalline Fe oxides such as lepidocrocite or goethite, iron oxides known to have decreased sorption capabilities. Conversely, the shift to oxic environments can inhibit this transformation by oxidizing dissolved Fe²⁺. The simultaneous effects of oscillating redox conditions on both phosphate sorption dynamics and the formation of secondary iron minerals remain largely unexplored.

Our research examined phosphate sorption under fluctuating redox conditions at circumneutral pH using synthetic ferrihydrite (25mM) at different phosphate levels (0, 0.05 and 0.5 mM).  Aqueous phosphate and Fe2+ concentrations as well as changes in Fe mineralogy were tracked over five redox cycles using photometric, spectroscopic and microscopic techniques.

Our preliminary data demonstrate that the ferrihydrite underwent partial transformation to lepidocrocite in all the experimental setups involving different phosphate treatments. Interestingly, even low phosphate concentrations were successful in substantially hindering the formation of goethite. The X-ray diffraction analyses affirmed the absence of any further Fe mineralogy transformations induced by recurrent redox oscillations. We observed a systematic reduction in the dissolved phosphate levels during the anoxic phases, a trend hypothesized to be driven by vivianite precipitation alongside sorption reactions.

This work underlines the role of phosphate in stabilizing low-crystalline Fe oxides against transformations, thereby playing a pivotal role in altering the pathways of phosphate immobilization in environments with fluctuating redox conditions.